(1) Field of the Invention
This invention relates to a long wave UV ray absorber and more particularly to a long wave UV ray absorber containing a dibenzoylmethane derivative and also to a cosmetic composition comprising such absorber.
(2) Description of the Prior Art
It is known that UV ray brings about a variety of changes to the skin. Dermatologically, the UV ray is classified into a long wave UV ray of 400 to 320 nm, an intermediate wave UV ray of 320 to 290 nm and a short wave UV ray of below 290 nm, which are, respectively, called UV-A, UV-B and UV-C.
The most common UV ray source to which one is exposed is usually the sunlight. UV rays which reach the ground are UV-A and UV-B, and UV-C does rarely reach the ground because it is absorbed in the ozone layer. Among the UV rays which can reach the ground, the UV-B, if irradiated on the skin in an amount more than a certain level, red spots or blisters are formed. In addition, melanosis is promoted and the skin undergoes changes such as pigmentation. In contrast, UV-A has been considered not to cause a substantial change in the skin. However, it has been recently revealed by electron microscopic and histochemical techniques that the skin suffers changes by UV-A irradiation. Especially, unlike UV-B, UV-A allows its energy to reach the true skin and brings about slightly chronic changes in the blood walls and the elastic fibers of connective-tissue membranes. These changes are considered to accelerate the ageing of the skin. It is also known that immediately after irradiation, UV-A acts to cause melanism of the skin (instant melanism) and can promote the degenerating action of UV-B on the skin. Thus, it is considered that UV-A is one of the factors which cause spots or freckles to be formed or exacerbated.
As will be apparent from the above, it is considered to be important that the skin is protected not only from UV-B, but also from UV-A in order to prevent the promotion of ageing of the skin and the formation or exacerbation of spots or freckles.
However, studies on the action of UV-A on skin have just started and few substances capable of effectively absorbing UV-A are known. Currently known substances are dibenzoylmethane derivatives and cinnamic acid derivatives, most of which are liposoluble (West Germany Patent Application Laid-open Nos. 2728241 and 2728243, and Japanese Patent Application Laid-open Nos. 51-61641, 52-46056 and 57-197209), with only a few water-soluble substances being known (Japanese Patent Application Laid-open No. 57-59840).
The present inventors already proposed in Japanese Patent Application Laid-open No. 60-190708 dibenzoylmethane derivatives which satisfy the following requirements (1) to (8) for a lone wave UV ray absorber.
(1) Have a maximum absorption wavelength at approximately 350 nm.
(2) The molar absorptivity (.epsilon.) at the above maximum absorption wavelength is sufficiently high.
(3) Since coloration is not favorable upon use as a cosmetic composition, an absorption in a visible light range has to be small, e.g. .epsilon..apprxeq.0 over 400 nm.
(4) High heat and light stabilities.
(5) No toxicity, irritativeness and other harmful actions on skin.
(6) Good compatibility with cosmetic base materials.
(7) Being sparingly absorbed percutaneously and removed by perspiration, i.e., having a prolonged efficacy.
(8) Inexpensiveness.
The derivatives are of the following formula (II) ##STR1## in which X represents a hydrogen atom, a monovalent metal cation, an organic cation, a linear or branched aliphatic hydrocarbon group having from 1 to 24 carbon atoms or a polyoxyethylene or polyoxypropylene oxide group, Ys and Zs may be the same or different and represent a hydroxyl group, a linear or branched aliphatic hydrocarbon group having from 1 to 24 carbon atoms, an alkoxy group having from 1 to 24 carbon atoms or a polyoxyethylene or polyoxypropylene oxide group, and m and n are independently an integer of from 0 to 3.
However, known UV ray absorbers are disadvantageous in that they are dissolved in water, organic solvents, oil substances, sebum and the like, and are percutaneously absorbed or diffused over the skin, so that problems are involved in the safety on the human body and the lasting efficacy. In order to solve the problems, there have been proposed polyvalent metal salts of paramethoxycinnamic acid (Japanese Patent Application Laid-open No. 60-94936) and polyvalent metal salts of paradimethylaminobenzoic acid (Japanese Patent Application Laid-open No. 60-94949). However, these compounds do not absorb UV-A and are not satisfactory in protecting the skin from UV rays.
Accordingly, there is a demand for a long wave UV ray absorber (hereinafter referred to as UV-A absorber) which has an intense UV-A absorption action and overcomes the above problems.